The present invention relates to an active matrix addressing type liquid crystal display device using thin film transistors (TFTs) and a method of manufacturing the same.
An active matrix addressing type liquid crystal display device includes switching elements each being provided correspondingly to one of a plurality of pixel electrodes arranged in a matrix. The active matrix addressing type display has a feature that liquid crystal at each pixel is theoretically driven at all times and accordingly it is higher in contrast than a simple matrix type adopting time-multiplexed driving. Such an active matrix addressing type display has become essential, particularly for color display.
A TFT arrangement used for a conventional active matrix addressing type liquid crystal display device includes a scanning signal line (gate line) formed on a transparent insulating substrate; a gate insulator formed on the scanning signal line; a semiconductor layer formed on the gate insulator; and a drain electrode (data line) and a source electrode formed on the semiconductor film, wherein the source electrode is connected to a transparent pixel electrode and the drain electrode (data line) is supplied with a video signal voltage. A TFT structure of a type in which a gate electrode is formed directly on a substrate is generally called an inverted staggered structure. Such a TFT is known from Japanese Patent Laid-open No. Sho 61-161764.
The liquid crystal display device using TFTs enables active addressing and thereby it exhibits high contrast; however, it is complicated in formation of TFTs on a substrate and also requires six or more photolithography steps. This is disadvantageous in terms of increasing manufacturing cost of a TFT substrate and decreasing processing yield with the increased number of manufacturing steps due to dust or dirt.
A method for simplifying manufacturing steps has been proposed, wherein a gate insulator, a semiconductor layer, and a metal film (for drain and source electrodes) are formed; the semiconductor layer is processed using the metal film as a mask; and a transparent electrode is formed. This prior art, however, is disadvantageous in that in the case where the metal film forming the source electrode is smaller in etching rate than the semiconductor film, the source electrode overhangs and increases open line defect probabilities for the transparent electrode due to the presence of a step at the overhang. In other words, in the above prior art the manufacturing yield has never been considered sufficiently.
It is required to increase the size of an area of a transmitting portion (hereinafter, referred to as an aperture ratio) of a transparent pixel electrode for realizing a bright display screen. The above-described prior art, however, failed to improve the aperture ratio for obtaining a display of high contrast and low cross talk.
An object of the present invention is to provide an active matrix addressing type liquid crystal display device capable of obtaining a display screen of high contrast and low cross talk.
Another object of the present invention is to provide a method of making an active matrix addressing type liquid crystal display device, which is capable of reducing the number of manufacturing steps and increasing a manufacturing yield.
To achieve the above objects, according to one preferred embodiment of the present invention, there is provided a liquid crystal display device including a plurality of scanning signal lines extending in a direction on a substrate; a first insulating film covering the plurality of scanning signal lines; a plurality of data lines extending on the first insulating film in such a manner as to intersect the plurality of scanning signal lines; a plurality of transparent pixel electrodes disposed at intersections of the plurality of scanning signal lines and the plurality of data lines; a plurality of thin film transistors each associated with one of the plurality of pixel electrodes, an output electrode thereof being connected to one of the plurality of pixel electrodes, a control electrode thereof being connected to one of the plurality of scanning signal lines, and an input electrode thereof being connected to one of the plurality of data lines; and a second insulating film disposed between the plurality of pixel electrodes and the plurality of thin film transistors; wherein a semiconductor layer is interposed between the first insulating film and a portion of the plurality of data lines, and wherein edges of the semiconductor layer are set back inwardly from edges of the plurality of data lines.
According to another preferred embodiment of the present invention, there is provided a method of making a liquid crystal display device, the liquid crystal display device including a plurality of scanning signal lines extending in a direction on a substrate; a first insulating film covering the plurality of scanning signal lines; a plurality of data lines extending on the first insulating film in such a manner as to intersect the plurality of scanning signal lines; a plurality of transparent pixel electrodes disposed at intersections of the plurality of scanning signal lines and the plurality of data lines; a plurality of thin film transistors each associated with one of the plurality of pixel electrodes, an output electrode thereof being connected to one of the plurality of pixel electrodes, a control electrode thereof being connected to one of the plurality of scanning signal lines, and an input electrode thereof being connected to one of the plurality of data lines; and a second insulating film disposed between the plurality of pixel electrodes and the plurality of thin film transistors; wherein a semiconductor layer is interposed between the first insulating film and a portion of the plurality of data lines; the method comprising the steps of: etching a transistor semiconductor layer underlying the output electrode in the plurality of thin film transistors using a photoresist mask formed on the transistor semiconductor layer such that an etched end of the transistor semiconductor layer on a side of the output electrode extends beyond an end of the output electrode; and etching an end of the semiconductor layer underlying the plurality of data lines using a metal film making up the plurality of data lines as a mask.